Conventionally, in performing mutual data transmission between a plurality of electronic control units (hereinafter referred to as the "ECU's") via a common communication line (hereinafter referred to as "the network bus") connecting between the ECU's, there is known a method (token passing method) which circulates a transmission right between the ECU's in a predetermined sequence to permit only the ECU having received the transmission right to send out data to the network bus.
In a data transmission system employing the token passing method, if the first ECU to acquire the transmission right e.g. when the power is turned on, is fixed, there arises a problem that the circulation of the transmission right is not started if the first ECU does not become operative from some cause. To solve this problem, a data transmission system has been proposed by Japanese Provisional Patent Publication (Kokai) No. 63-88930, in which a plurality of ECU's are set to respective waiting time periods, and when a waiting time period set for one ECU has elapsed after the power is turned on, the one ECU acquires the transmission right if the other ECU's is not then sending out data to the network bus.
According to the proposed system, the waiting time periods for the respective ECU's have to be set to fairly different values from each other to avoid overlapping of the data transmission timing, since the ECU's are different in rise time, i.e. a time period to elapse from the time the power is turned on (or the system is reset) to the time each ECU actually becomes operative. This causes a problem that a long time period elapses before generation of the transmission right (acquisition of the transmission right by any one of the ECU's).
Further, even if the transmission right is generated after the power is turned on, if an ECU having the transmission right ceases to operate, the transmission right disappears without being sent to any other ECU. The above proposed system does not take this disadvantage into consideration, and hence cannot cope with such a problem.
The present invention has been made in view of the problems described above, and it is the object of the invention to provide a data transmission system which is capable of generating the transmission right in a short time both when the power is turned on and when the transmission right has disappeared.